1. Field of the Invention
The present invention relates to an electromagnetic-wave (EMW) oscillator. Particularly, the present invention relates to an EMW oscillator for emitting an electromagnetic wave at a frequency or frequencies including at least a portion of a frequency range from 30 GHz to 30 THz. Electromagnetic waves including at least a component in the above frequency range are called terahertz (THz) waves in this specification.
2. Description of the Related Background Art
In recent years, techniques using a THz wave in communications field, security field, medical field, and so forth have been energetically researched and developed. The THz wave has characteristics of penetrating and straightly propagating through a substance. It is, therefore, possible to obtain inner information of a substance at high resolution by using a THz wave reflected by, or transmitted through, the substance. Accordingly, various non-destructive or non-invasive inspection techniques have been researched and developed.
Some examples of these inspection techniques include a technique for securely performing a seeing-through or perspective imaging of a substance using a THz wave in place of X-rays, a spectroscopic technique for inspecting the bonding condition of a molecule by obtaining the absorption spectrum and the complex dielectric constant of a substance, a technique for estimating the carrier density and the mobility of a superconductive material, a technique for analyzing a bio-molecule, such as DNA and proteins, etc.
The development of a THz-wave source is indispensable to put the above techniques into a practical use. To date, there have been developed THz-wave generating techniques using laser apparatuses, such as a photoconductive device capable of being excited by a femtosecond (fsec) laser light, and a THz-wave parametric oscillator using a non-linear optical crystal. Further, there have also been developed THz-wave generating techniques using a small-sized electron vacuum tube, such as a backward-wave oscillator (BWO) and a gyrotron, and a large-sized electron beam accelerator, such as a free electron laser. According to those techniques, the oscillating frequency is changeable, and the output power is large, so that those techniques are highly effective in particular uses, such as the identification of fingerprint spectra of various substances. However, in those techniques, the size of an apparatus increases, and hence its general or industrial use is restricted.
The following oscillators have been developed as a small-sized radiation source. For example, some oscillators are constructed by combining active elements using the negative resistance generated by the movement or transition of electrons in a semiconductor due to injection of a current thereinto, such as a Gunn diode and a resonance tunnel diode (RTD), with a variety of antennas (or resonance structures). In this way, a small-sized oscillator with a single oscillation frequency can be realized, though its output is low (especially low in an oscillator for emitting a THz wave). Therefore, such oscillators are expected to be applied to various uses. Conventional examples of such a small-sized oscillator will be described in the following.
“APL, Vol. 58 (20), p. 2291, 1991” discloses a small-sized oscillator constructed by the combination of an active element of a double barrier RTD with AlSb barrier layers (1.5 nm in thickness) and InAs quantum well layers (6.4 nm in thickness) grown by a molecular beam epitaxy (MBE), and a square (300 microns×150 microns) waveguide serving as a resonance structure. According to this referenced paper, oscillation at a frequency of 712 GHz is achieved by a single device, and its output is 0.3 microwatt.
Further, “IEEE, Transactions on microwave theory and techniques, Vol. 42, No. 4, 1994” discloses a planar integrated Gunn diode array constructed by the combination of an active element of a GaAs Gunn diode, and a microstrip line (MSL) patch antenna serving as a resonance structure.
FIG. 7 illustrates this planar integrated Gunn diode array. According to this referenced paper, it is possible to collectively fabricate the active element (negative resistance element), the resonance structure (antenna), and a DC (direct current) supplying portion (circuit and electrode for performing the DC supply to the active element) on a substrate, using a conventional semiconductor process. Accordingly, decrease in a size of the oscillator, increase in an output by arraying the structures, and improvement in oscillation characteristics are expected. According to this referenced paper, oscillation at 12.423 GHz is achieved in a Gunn diode of a 4×1 linear array type (illustrated in FIG. 7), and oscillation at 12.395 GHz is achieved in a Gunn diode of a 2×2 loop array type.
Furthermore, “Preliminary Papers at the 53rd JSAP (Japan Society of Applied Physics)-related lecture, 2006 spring, No. 3, 23p-M-2” suggests as follows. As a method for overcoming the problem of a low output power in a range of the THz wave, it is expected that the output power of an oscillator in a range of 330 GHz can be increased by mutually synchronizing current injections into densely arrayed oscillating elements.
In conventional THz-wave oscillators of the planar integrated type, it is necessary to arrange a DC supplying portion including elements, such as a circuit, an extraction electrode, and a bonding wire, on a substrate, in addition to an EMW radiating portion including elements, such as a negative resistance element, an antenna, an EMW resonance portion. The size of the DC supplying portion is in the order of approximately 1 mm2. It is therefore likely that the DC supplying portion occupies a considerable space, and the flexibility of design of the EMW radiating portion on the substrate is reduced. Further, the DC supplying portion and the EMW radiating portion are disposed on the same substrate, so a THz wave emitted from the radiating portion is likely to interfere with the DC supplying portion. Also in this respect, the flexibility of design tends to be restricted. As a result, it is not easy to densely array THz-wave oscillators of the planar integrated type.